Fallback functionality helps ensure notifications are delivered when incomplete client profile information exists. For example, if a client’s profile is missing an email or cellphone number.
It also kicks in when you run out of tokens.
This is how it works, if you're using, as an example, e-mail with SMS fallback:
if an email address is not available, but a cellphone number is available, an SMS will be sent
if an email address is available, and a cellphone number is available, an email will be sent
if an email address is available, and a cellphone number is not available, an email will be sent.
And vice versa.
Emails will also be sent out as fallback if you run out of SMS tokens.
Fallback messages will not be sent if delivery fails, for whatever reason. For example, if an SMS is not delivered because the number does not exist, an email will not be sent. And vice versa.